1. Field of the Invention
The present invention relates to controllable current source circuitry, and more particularly to controllable bipolar current sources operable to apply bipolar current across a load, such as a thermoelectric cooler, with a unipolar power supply.
2. Description of Related Art
Thermoelectric coolers (TEC) are solid state heat pumps used for a variety of cooling and heating applications, including heat removal and temperature control of thermal loads including laser diodes and laser crystals. The amount of heat transferred is adjusted by controlling the current through the TEC; cooling or heating functions are controlled by changing the direction of current flow through the TEC, so bipolar application of the current is required. As the operating power of said thermal loads rises, thermoelectric cooler assemblies used for these temperature control tasks may require several hundred watts of electrical power, with applied voltages of several tens of volts and currents of tens of amperes.
A bipolar current supply for driving a TEC may be assembled from an adjustable single-ended or unipolar source by adding electronically-controlled switches or conductive elements 5, 6, 7, and 8 in an H-bridge configuration shown in FIG. 1a. The magnitude of the voltage applied to the load 4 (or current passing through the load 4) is controlled by an input voltage Vsupply control 3 provided to the controlled power supply 1, while the polarity of the voltage applied to the load 4 is controlled by the sign of the input voltage Vsupply control 3 through the use of Control Logic 2. For example, if the input voltage Vsupply control 3 is positive with respect to ground, switches 6 and 7 are closed, making terminal 11 positive with respect to terminal 10. This circuit consisting of switches 5, 6, 7 and 8 and load 4 may be referred to as a “digital H-bridge” because the conductive elements 5, 6, 7 and 8 are either fully open or fully closed.
The action of the circuit in FIG. 1a may be graphically described in FIG. 1b, wherein the load current through 4 is plotted versus the voltage Vsupply control 3. For positive values of Vsupply control, switches 6 and 7 are closed and terminal 11 is positive with respect to terminal 10, while for negative values of Vsupply control, switches 5 and 8 are closed and terminal 10 is positive with respect to terminal 11. When the load 4 is a thermoelectric cooler, the polarity of terminal 11 with respect to the polarity of terminal 10 determines whether the thermoelectric cooler is used for heating or cooling. An example of this type of circuit utilized for the case in which the load 4 is a thermoelectric cooler is described in U.S. Pat. No. 5,936,987.
For the voltage-controlled power supply 1, it is highly desirable to utilize compact, pre-packaged, inexpensive and powerful switching power supplies which are available from commercial vendors. However, due to limitations in the design of said switching power supplies, they are often not adjustable over a full range of voltage output from zero to some maximum voltage, and may exhibit instabilities, or may not function at all when used below some particular minimum voltage Vminimum.
Referring to FIG. 2, an alternative to the above embodiment is to replace the voltage-controlled power supply 1 in FIG. 1 with a fixed-voltage supply 12, and replace switches 5, 6, 7 and 8 with voltage-controlled conductive elements 13, 15, 17 and 19 in which the current flow through said conductive elements depends linearly on a controlling voltage applied to terminals 14, 16, 18 and 20 of said conductive elements, said controlling voltage derived through the action of linear control circuitry 21 which is in turn further derived from an additional voltage Vcontrol 22. The conductive elements are activated in pairs diagonally across the H-bridge. By activating elements 15/17 as a pair, terminal 11 is made positive with respect to terminal 10. By activating elements 13/19 as a pair, terminal 10 is made positive with respect to terminal 11. One element out of each pair 15/17 or 13/19 may be a switch-like digital element that is either fully conducting or fully open. The circuitry of FIG. 2 (not including the fixed-voltage supply) may be referred to as a “linear H-bridge”. An example of such a linear bridge is described in the prior art of U.S. Pat. No. 6,023,193.
This circuit has the disadvantage of being inefficient, which is of special importance in very high current applications. For example, in the worst-case condition for which the voltage drop across the load 4 is one-half of the fixed supply voltage Vsupply 9, the power dissipated in the linear element is equal to that delivered to the load 4 and the circuit thereby provides an overall “line-plug” efficiency of no greater than 50%. This forces the use of high-power-dissipation devices for the controlled conductive elements and also necessitates the use of large heat sinking and cooling fan assemblies, increasing both the physical size and cost of the unit.